Since introduction of of_display_timings_exist() function in commit
cc3f414c ("video: add of helper for display timings/videomode") it
didn't attract any users, and the function has no potential, because
of_get_display_timings() covers its functionality and does more.

Drop the unused exported function from the kernel.
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>